Higher operating temperatures for gas turbine engines are continually sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase. Thus, the material capability to withstand higher temperatures must also increase.
Components formed from powder metal gamma prime (γ′) precipitation strengthened nickel-base superalloys can provide a good balance of creep, tensile and fatigue crack growth properties to meet performance requirements. Typically, a powder metal component is produced by consolidating metal powders in some means, such as extrusion consolidation, then isothermally forging the consolidated material to the desired outline, and finally heat treating the forging prior to machining to the final geometry. The processing steps of consolidation and forging are designed to retain a fine grain size within the material to promote superplasticity, so as to minimize die loading and improve shape definition. In order to improve the fatigue crack growth resistance and mechanical properties of these materials at elevated temperatures, these alloys are then heat treated significantly above their gamma prime solvus temperature, to cause uniform coarsening of the grains. For example, rotors, disks, shafts and disk-like seals for aircraft engine gas turbine applications are often manufactured from gamma prime precipitation strengthened nickel-base superalloy forgings. To improve temperature capability and component reliability, the forgings are solution heat treated at temperatures significantly above the gamma prime solvus temperature to yield an average grain size of about 90 μm to 16 μm (ASTM 4–9 (Reference throughout to ASTM grain sizes is in accordance with the standard scale established by the American Society for Testing and Materials)) often followed by precipitation heat treatment, including subsolvus stress relief and/or subsolvus aging heat treat. Cooling or quenching from the above solution heat treatment process introduces residual stresses in the component. Although a minor amount of the as-quenched stress may be relieved during the precipitation heat treat exposure, often in the 1400–1550° F. (760–815° C.) range, residual stress in the resultant heat treated forgings affects component manufacturing cost and may degrade component reliability during engine operation.